Blood pressure is a physiologic parameter that is often measured to gain an understanding about the condition of a person's cardiovascular system and overall health. Arterial blood pressure, referred to simply as blood pressure, is caused by the pumping action of the left ventricle of the heart and the resistance to flow caused by the vascular system. When the heart beats or “pumps”, blood is forced through the arteries to the capillaries. Blood pressure is typically characterized by two readings, the systolic and diastolic pressures. The systolic pressure is the pressure as the heart contracts and is the higher of the two pressures. The diastolic pressure is the pressure when the heart relaxes and fills with blood in preparation for another contraction.
In a typical measurement of blood pressure a cuff is secured around a patient's limb and is inflated to a sufficiently high pressure to cut off arterial blood flow beneath the cuff and then the cuff is gradually deflated to allow the artery to slowly open. As the cuff is deflated, blood is able to pass through the blood vessel beneath the cuff. As the blood is able to pass through, sounds, known as Korotkoff sounds, are created and form biological signals indicative of blood pressure. These sounds can be detected by a trained clinician using a stethoscope to determine a person's blood pressure. This is known as an occlusive measurement because the artery is occluded (pressed shut) for a brief period of time during the measurement.
An alternative non-invasive way to determine blood pressure on the basis of sounds is to detect the Korotkoff sounds by a transducer. When the blood pressure sounds are detected by a transducer they are converted into an electrical signal that is processed to determine the systolic and diastolic pressure. Other types of non-invasive techniques to determine blood pressure are known. For example, an oscillometric technique measures the pressure change in a cuff induced by flowing blood and converts the pressure change to an electrical signal which is used as the basis for determining blood pressure. Another procedure involves using multiple transducers to detect the occurrence of heart pulses at different locations along the artery. In this example mean blood pressure is determined by the pulse propagation time between the transducers.
Blood pressure can be accurately determined by using a catheter that is fitted with a pressure transducer and inserting the catheter into an artery. The pressure that is measured is direct and accurate. The measurement is also continuous. However, this technique has the disadvantage of being invasive. That is, to obtain blood pressure reading this way, a patient must have a puncture through the skin in order to deliver the catheter into the blood vessel.
Often it is clinically desirable to measure blood pressure during critical care periods of a patient. For example, during surgery it is desirable to know a persons blood pressure so that the anesthesiologist can monitor the patients. It is advantageous that the measurement be continuous because of the risks to the patient during surgery. An occlusive measurement cannot be repeated with the frequency that may be desired in the operating room. Additionally, patient monitoring before and after surgery typically includes monitoring the blood pressure. Currently, there is no single reliable non-invasive measurement device or technique that provides a clinician with continual blood pressure information that is desired.